This invention relates to electronic instruments and techniques for measuring electrical signals and, more particularly, to electronic instruments and techniques for measuring the noise figure of a device under test. Specifically, one embodiment of the invention provides an apparatus and method for determining the single sideband noise figure of a device under test from double sideband measurements.
Measurement of noise figure at radio (e.g., microwave) frequencies typically requires the use of a local oscillator (LO) and mixer to down-convert the measurement frequency to a suitable intermediate frequency (IF) for detection by a receiver. During conventional noise figure measurements on a device under test, a radio frequency (RF) input signal from the device under test is down-converted by the mixer. The mixer combines the RF input signal with a signal produced by the LO, or a harmonic of the signal from the LO, to produce a predetermined IF output signal at a frequency suitable for further processing by the receiver. This causes the down-conversion of an image signal as well as the wanted signal. The resultant noise power at the intermediate frequency is typically assumed to be an average of the power in the two sidebands. However, this image signal can be a source of considerable measurement error when the device under test does not have a flat frequency response. See, "Noise Figure Measurement Accuracy," Hewlett-Packard Application Note 57-2.
The graph in FIG. 1 illustrates the result of the down conversion by the mixer, evidencing the relationship between the LO, RF, and predetermined IF frequencies. In FIG. 1, the vertical axis represents signal power, and the horizontal axis represents signal frequency. The predetermined IF signal 25 has a frequency equal to the difference between the LO signal (or harmonic) 27 and the RF input signal 29, so that-the RF input signal is measured by monitoring a set IF frequency, below the LO signal frequency, at f.sub.RF =(n)f.sub.LO -f.sub.IF. However, an image RF signal above the LO signal frequency, at f'.sub.RF =(n)f.sub.LO +f.sub.IF, will also produce a signal at the monitored IF frequency. To resolve this ambiguity, an analog bandpass filter is typically provided over a frequency range including f.sub.RF, as shown by the broken line curve 31 that appears in FIG. 1, thereby attenuating any image signal 33 at f'.sub.RF. The bandpass filter is essential to eliminate unwanted mixing products from being measured.
The conventional technique to remove image noise is to filter the signal from the device under test before the signal is down-converted. The passband of the bandpass filter must track the sweeping LO signal, with the center frequency of the passband separated from the LO frequency (or harmonic) by the IF signal frequency when noise figure measurements are performed over a range of frequencies to eliminate the unwanted sideband. This requires a suitable filter and for broadband measurements requires that either a tunable bandpass filter, such as a magnetically tunable yttrium-iron-garnet (YIG) or barium-ferrite filter, or fixed tuned filters with multiple conversions be used. However, this can be time-consuming and costly, because either a number of filters with different passbands or a very expensive tunable filter is required.
Furthermore, an analysis of frequency-conversion techniques has previously been applied to microwave transistor noise measurements. See, G. Caruso and M. Sannino, "Analysis of Frequency-Conversion Techniques in Measurements of Microwave Transistor Noise Temperatures," IEEE Trans. on Microwave Theory and Techniques, vol. MTT-25, no. 11, pp. 870-873, November 1977, and G. Caruso and M. Sannino, "Determination of Microwave Two-Port Noise Parameters Through Computer-Aided Frequency-Conversion Techniques," IEEE Trans. on Microwave Theory and Techniques, vol. MTT-27, no. 9, pp. 779-783, September 1979. However, the reported techniques only correct for changes in noise figure due to changes in the noise source impedance between the two sidebands. Also, some of the applied assumptions are not applicable to all possible test devices.
It is desirable that an alternate technique to analog filtering for removing the noise power at the image frequency be provided when single sideband noise figure measurements are performed on a device under test. Furthermore, it is desirable that these single sideband noise figure measurements be corrected for change in output impedance of the noise source and noise source impedance mismatch with the receiver.